In recent years, there has been a demand for high wet grip performance and low rolling resistance in pneumatic tires. In order to satisfy these demands, there is a known technique of compounding a reinforcing filler such as a styrene-butadiene copolymer or silica with a rubber composition constituting a cap tread of a tire. In order to further enhance the wear resistance or rubber hardness of the rubber composition, it has been proposed to compound polybutadiene or a silica having high reactivity, for example, but in this case, there has been a problem in that the rubber strength decreases or the processability is diminished.
Japanese Unexamined Patent Application Publication No. 03-239737 describes that a pneumatic tire using a rubber composition prepared by compounding a silica and a styrene-butadiene copolymer having a specific arrangement of styrene-derived units for a tread simultaneously achieves wet skid resistance, rolling resistance, and wear resistance. However, this rubber composition could not necessarily satisfy the demands of consumers sufficiently due to a decrease in rubber strength. The rubber composition also could not enhance rubber hardness sufficiently.
Japanese Unexamined Patent Application Publication No. 57-179212 describes a styrene-butadiene copolymer in which a long-chain styrene block content is not greater than 5 wt. %, a simple chain content having one styrene-derived unit is not less than 50 wt. % relative to the total styrene content in the styrene-butadiene copolymer, and a total styrene content is from 10 to 30 wt. % of the styrene-butadiene copolymer content. However, the rubber composition described in Japanese Unexamined Patent Application Publication No. 57-179212 could not necessarily improve rubber hardness sufficiently.